Sounds like COMET is indeed the solution you re looking for. In that scenario, you don t need to poll, nor do comparisons, as you can push out only the "relevant" changed data to your grid.

Check out WebSync, it s a nice comet server for .NET that ll let you do exactly what you ve described.

Here s a demo using ExtJS and ASP.NET that pushes a continuous stream of stock ticker updates. The demo is a little more than you need, but the principal is identical.

Every time you get the answer from the server, check if something has changed.

Do a request. Do let the user know that you are working with some spinner, don t hide it. Schedule the next request in 15 seconds. The next request executes; if nothing has changed, schedule the next one in 15 + 5 seconds. The next request executes; if nothing has changed, schedule the next on in 15 +5 +5 seconds. And so on. The next request executes; if something has indeed changed, reset the interval to 15 seconds.

Prototype can do this semi-automatically with Ajax.PeriodicalUpdater but you probably need stuff that is more customized to your needs.

Anyway, just an idea.

As for continuous polling in general; it s bad only if you hit a different site (using a PHP "bridge" or something like that). If you re using your own resources you just have to make sure you don t deplete them. Set decent intervals with a decay.

I suggest Comet is not an overkill if "updates need to be constant." 15 seconds is very frequent; is your visited by many? Your server may be consumed serving these requests while starving others.

I don t know what your server-side data source looks like, or what kind of data you re serving, but one solution is to server your data with a timestamp, and send a timestamp of the last poll with every subsequent request.

1. Poll the server, sending the timestamp of when the service was last polled (eg: lastPollTime).
2. The server uses the timestamp to determine what data is new/updated and returns only that data (the delta), decreasing your transmission size and simplifying your client-side code.
3. It may be empty, it may be a few cells, it may be the entire grid, but the client always updates data that is returned to it because it is already known to be new.

The benefits of this method are that it simplifies your client side code (which is less code for the client to load), and decreases your transmission size for subsequent polls that have no new data for the user.

Also, this allows you to maintain state on the server side because you don t have to save a state for each individual user. You just have one state, the state of the current data, that is differentiated by access time.

I think checking if there is any new data is a good option.

I would count the number of rows in the database and compare that with the number of rows in your (HTML) table. If they re not the same, get the difference in rows.

Say you got 12 table rows and there are 14 database rows as you check: Get the latest (14 - 12) = 2 rows.